The U.S. Justice Department today sued Fiat Chrysler Automobiles, charging it with building and selling diesel engines that violate the Clean Air Act.

The automaker faces four claims in the suit that could result in billions of dollars in penalties.

The federal government said Fiat Chrysler created software that acted as a defeat device for emissions testing on 104,000 Ram pickups and Jeep Grand Cherokee SUVs. Such devices would be designed to "reduce the effectiveness" of the emissions control system under normal driving conditions, apparently outside of vehicle testing.

The lawsuit comes after months of negotiations and discussions between the Auburn Hills automaker, the U.S. Environmental Agency, the California Air Resources Board, which regulates vehicles emissions in California, and federal investigators.

Just last month, FCA CEO Sergio Marchionne told Wall Street analysts he was hopeful the matter would be resolved "in the next few weeks."

But today, the federal government sued the company.

"The complaint alleges that FCA equipped nearly 104,000 Ram 1500 and Jeep Grand Cherokee vehicles (Model Years 2014-2016) sold in the United States with at least eight software-based features that were not disclosed in FCA’s applications for certificates of conformity and that affect the vehicles’ emission control systems," the EPA said in a statement. "The undisclosed software features lessen the effectiveness of the vehicles’ emissions control systems during certain normal driving situations. This results in cars that meet emission standards in the laboratory and during standard EPA testing, but during certain normal on-road driving emit oxides of nitrogen (NOx) that are much higher than the EPA-compliant level," according to an EPA release.

The resulting pollution "contributes to the formation of harmful smog and soot, exposure to which is linked to a number of respiratory- and cardiovascular-related health effects as well as premature death," the complaint said.

Fiat Chrysler said it was disappointed in the lawsuit.

"FCA US is currently reviewing the complaint, but is disappointed that the (Department of Justice Environment and Natural Resources Division) has chosen to file this lawsuit. The Company intends to defend itself vigorously, particularly against any claims that the Company engaged in any deliberate scheme to install defeat devices to cheat U.S. emissions tests," the company said in a release.

While there are similarities between the Fiat Chrysler and Volkswagen cases, Volkswagen acknowledged that it intentionally designed devices in its diesel vehicles to defeat emissions testing, but Fiat Chrysler has maintained that it did not. Marchionne has also pushed back against comparing the two cases.

Volkswagen was accused in September 2015 by the federal government of willfully creating software designed to cheat on emissions tests. Those allegations led Volkswagen to agree to more than $17 billion in civil settlements as well as a $2.8-billion criminal fine.

Today's action is in contrast to Fiat Chrysler's position in April when Marchionne told Wall Street analysts that the automaker and federal officials were working in a "collaborative spirit" to resolve the issue.

"We have found a very collaborative spirit on the part of the regulators to try and get us through the hurdles," he said at the time. "We keep on providing data and adjusting our submissions to reflect their concerns.I'm hopeful that somehow in the next few weeks we should be able to resolve the issue."

Last week, the automaker formally applied for emissions certification for its 2017 Jeep Grand Cherokee and Ram 1500 diesel vehicles that it hopes will resolve the certification issues.

The automaker believes the software fix it has proposed will resolve concerns from the federal government and the California Air Resources Board and clear the way for it to finally sell 2017 diesel SUVs and pickups, which it has been prevented from doing since January. The automaker wants to use that fix to address the issues with the older models.